The invention relates generally to processes wherein a fluid which forms incrustations undergoes temperature change. Of particular interest is the extraction of bauxite with sodium aluminate lye.
It is already known to continuously extract bauxite from a suspension which also contains sodium aluminate lye by passing the suspension through a conduit under pressures of 10-200 atmospheres, at temperatures of 150.degree.-300.degree. C and under conditions of turbulent flow at flow speeds of 0.5-7 meters per second. Preferred flow speeds are between 2 and 5 meters per second.
As the suspension is pumped through a conduit under high pressure in an apparatus provided for this purpose, it passes through heat-exchangers into which vapors are admitted from devices provided for expansion of the suspension. These heat-exchangers are followed by one or more additional heat-exchangers which are heated with high-pressure steam, diphenyl vapors, a liquid organic heat carrier, a salt melt or the like.
In the known apparatus, it is known to physically dimension all of the heat-exchangers in accordance with the quantity of heat which is to be transmitted. This quantity of heat is selected so as to be the same everywhere along the length of the conduit and, consequently, the heat-exchangers are all of the same physical dimensions. Quite surprisingly, however, it has been found that the formation of incrustations, which latter result from the suspension, differs extremely in the regions of the different heat-exchangers. As a result of this, the heat exchanged in the individual heat-exchangers varies greatly because the heat-exchangers are all of identical dimensions. This, in turn, has the result that the desired temperatures in the individual conduit sections provided with these heat-exchangers cannot be maintained over an extended period of time so that the operation of the apparatus is not at its optimum. Due to the fact that the apparatus does not operate at the optimum, the passage time of the suspension becomes too short to insure proper heating thereof or, to avoid this, it becomes necessary to make the apparatus of larger dimensions than would inherently be required if proper operation could be assured.